Systematic color variation across the solid solution members, Li₂Mn_1-xTi_xO₃ (0.0 ≤ x ≤ 1.0)

This study investigated the formation of a layered rock salt superstructure series for the oxides Li₂(Mn_1-xTi_x)O₃ (0.0 ≤ x ≤ 1.0) and the effects of the progressive substitution of Ti⁴⁺ (d⁰) for Mn⁴⁺ (d³) on the structural and optical properties. The members where 0.0 < x ≤ 0.85 adopted the structure of Li₂MnO₃ (S.G. C2/m; a = 4.934 (1); b = 8.530 (2); c = 5.021 (1) Å; β = 109.46 (1)°) according to powder X-ray diffraction measurements and the characteristic Raman bands. By contrast, the members corresponding to the range of 0.85 < x < 1.0 resembled that of Li₂TiO₃ (S.G. C2/c; a = 5.0585 (2); b = 8.7780 (4); c = 9.7452 (4) Å; β = 100.183 (3)°). Starting from the brick-red color of Li₂MnO₃, the color changed to ochre and then to orange as the concentration of Ti⁴⁺ increased, and finally to colorless for Li₂TiO₃. The corresponding photoluminescence spectrum for the lowest (0.05) Mn⁴⁺ substitution in Li₂TiO₃ confirmed the presence of Mn⁴⁺ ions, where the bands at 684 and 698 nm were due to the transitions: ²E → ⁴A₂ and ²T₁→⁴A₂. The photoluminescence intensities decreased systematically as the Mn⁴⁺ content increased starting from x = 0.95. The ionic conductivity measurements indicated the temperature dependence of the nominal conductivity values.